A bus system provides a structure including communication paths over which a plurality of functional units may communicate signals among one another. It is typically contemplated that a common set of signal lines ("party lines") will be used, with the various functional units being connected in parallel to the party lines. The bus system may also include a number of "private" lines that provide communication paths between particular functional units. Although the term "bus" is sometimes used to refer to the signal lines themselves, or perhaps only the party lines, the term is often taken to include, in addition to the signal lines, the interface electronics ("ports") associated with the functional units for transmitting and receiving signals according to a specified bus protocol. The bus system also typically comprises centralized or distributed logical units for arbitrating requests from functional units for use of the bus.
The signal lines are sometimes implemented as generally parallel traces on a printed circuit board, known as a "backplane," with the circuit boards for the functional units plugging into connectors that hold the functional unit boards perpendicular to the plane of the backplane and perpendicular to the direction of the backplane circuit board traces. The signal lines define, along with a nearby power plane at a fixed DC level (for example, ground), respective transmission lines along which the bus signals propagate. A so-called "differential" bus dedicates a pair of lines to a given signal, with a voltage difference between the lines defining the signal polarity.
As attempts are made to increase the speed of data communication on the bus, the travel time of the signals along the backplane becomes an increasingly significant factor. For example, if a differential bus is used, differential delay between lines of a line pair results in distortion of the data. Moreover, care must be taken to avoid spurious signals arising from reflections that occur at impedance discontinuities. Such discontinuities are present, for example, where the signal driver for a given functional unit is characterized by a low output impedance when it is in its sending state. Thus, it is necessary to wait for the signal level change resulting from a first device's turning off its bus drivers to propagate past the bus drivers of a second device before allowing the second device to enable its bus drivers to transmit on the bus. Alternately, if the second device is allowed to enable its bus drivers so as to cause reflections, it is necessary to wait until such reflections propagate past the intended recipient of the second device's transmission before allowing the intended recipient to receive the transmission. In either event, bus "turnaround" requires extra " settling time" which slows down the system.
Accordingly, while the use of a backplane bus is attractive from a convenience and economy standpoint, the above limitations on speed have tended to make backplane buses unsuitable for large, high-speed computer systems.